1. Technical Field of the Invention
The present invention relates to the recovery of ethane compounds from hydrocarbon gas streams. More particularly, the present invention relates to the recovery of ethane compounds from hydrocarbon inlet gas streams using multiple reflux streams.
2. Description of Prior Art
Many prior art processes exist for the recovery of ethane compounds from hydrocarbon inlet gas streams. An example ethane recovery process can be found in U.S. Pat. No. 5,890,377 issued to Foglietta. Residue recycle processes are capable of obtaining high ethane recoveries (in excess of 95%), while recovering essentially 100% of C3+. Such processes, though impressive in achieving high recoveries consume a lot of energy in terms of compression. In order to reduce energy consumption while still maintaining high recoveries, an additional source of reflux is required. The requirements for this reflux stream are that it should be lean in desirable components (C2+) and be available at a high pressure. Prior art schemes have identified some alternate sources of reflux. The process disclosed here has a unique way of obtaining such a reflux stream. This reflux stream is used as intermediate reflux thereby reducing flow of the main reflux stream and hence energy consumption. In this process, an inlet gas is cooled by heat exchange with other streams in the process, without first splitting the inlet gas stream. As the inlet gas stream is cooled, liquid can be condensed and separated to form a first liquid stream and a first vapor stream. The first vapor stream is expanded in a turboexpander to further cool the stream. The cooled stream is then introduced to a demethanizer column at an intermediate feed position. The first liquid portion from the separator is expanded and directed to the demethanizer at a relatively lower feed position. The overhead stream from the demethanizer is heated, and compressed to a higher pressure and then divided into a volatile gas residue fraction and a compressed recycle stream. The compressed recycle stream is cooled sufficiently to substantially condense it by contacting it with the side reboilers as a part of the demethanizer column. The compressed recycle stream is further cooled and expanded to a lower pressure and supplied to the demethanizer column at a top feed position to reflux the column. The Foglietta process described above achieves a relatively high recovery efficiency of 95% and greater for ethane and heavier compounds.
A need exists for an ethane recovery process that is capable of achieving a recovery efficiency of at least 95%, but with lower energy consumption compared to prior art processes. A need also exists for a process that can take advantage of temperature profiles within a process to reduce the amount of external energy requirements that are needed to achieve high recovery efficiencies.